“Extreme” marine heatwave in northeast Atlantic risks “massive die-offs”

    The European Space Agency has recorded sea temperatures up to 5°C higher than average, posing significant risks to fisheries and aquaculture.

    The seas around the UK and Ireland are experiencing an unprecedented marine heatwave, rated as “an extreme Category IV/V Marine Heatwave” by both the European Space Agency (ESA) and the US National Oceanographic and Atmospheric Administration (NOAA).

    Satellite measurements by the ESA have established that water temperatures in the North Sea are more than 5°C higher than the average during this time of year.

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    Responding to a request from WeAreAquaculture, marine heatwave expert Dr Dan Smale of the UK’s Marine Biological Association stated that marine heatwaves “do occur from time to time,” but “the heat content of the North Atlantic is at a record high. The amount of energy in the system means that contemporary heatwaves are likely to be amongst the highest on record.”

    What is causing the marine heatwave?

    “The drivers of this phenomenon are complex but likely to include weaker trade winds leader to lower evaporation and more uptake of solar radiation,” Smales explained.

    “The warming events occur naturally and are part of the complex ocean climate system. However, when they occur now, they are superimposed onto 100 years of significant ocean warming, so the starting point is much higher – around 1°C on average in the North Atlantic. We are recording temperatures in excess of 18°C, when the average sea temperature for June around the UK is around 12-14°C.”

    The NOAA also notes that the combination of the El Niño weather phenomenon, coupled with lower than normal levels of Sahara dust in the atmosphere, which would normally block solar radiation, has contributed to the heatwaves.

    Consequences of marine heatwaves for fisheries and aquaculture

    “It very complex and hard to predict, but we do know that marine species have evolved over millions of years to occupy specific thermal windows. When temperature thresholds are exceeded this can lead to high stress, reproductive failure and widespread mortality, with implications for foodwebs and the entire ecosystem,” Smale said.

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    “We’ve seen massive die-offs of kelp, seagrass, corals, fish, mammals and sea birds from marine heatwaves elsewhere, such as the Mediterranean, California and the west coast of Australia. Marine life around UK and Ireland is adapted to colder water and some species will become stressed by this heatwave if it persists for too long.”

    “If it continues through summer and actual temperatures start to exceed 20°C we might seen mortality events and shifts in ecosystem structure, with both winners and losers of the disturbance event. Currently, warm temperatures are likely to affect food supply, migration patterns and reproductive output,” he added.

    At present, it is unclear how the heatwave will affect aquaculture and fisheries in the area, however scientists warn that if the anomalously high temperatures continue for too long, this could trigger mass mortalities of fish, toxic algae and jellyfish blooms, and also extreme weather events.

    Last year’s heatwaves linked to biological problems and fish deaths

    In 2022, elevated temperatures were linked to a surge in fish deaths in the Scottish salmon farming industry. According to records published by Scotland’s Fish Health Inspectorate, the number of fish that died prior to harvest in 2022 was nearly double that of 2021 and triple of 2020, reaching approximately 16 million. The mortalities were largely attributed to blooms of micro-jellyfish, caused by the unusually high temperatures experienced last summer.

    Mowi Scotland recently experienced an unusual algae bloom at its site in Loch Ailort, but noted in an online blog that “fortunately it appears to be relatively harmless to our fish”.

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